A vehicle body side part structure includes a side panel outer member and a side panel inner member that are provided with at least one door opening. The side panel outer member and the side panel inner member are joined together so as to form a closed section. The side panel outer member is divided in the longitudinal direction of the vehicle at a portion located rearward of the door opening. A front section of the divided side panel outer member is formed of a high-tensile steel sheet (JIS-SPC590; plate thickness t=0.8 to 1.6 mm), and a rear section of the side panel outer member is formed of an ordinary steel sheet (JIS-SPC270; plate thickness t=0.6 to 0.75 mm).
“SPC” means cold rolled steel sheet. “SPC590” means a cold rolled steel sheet (high-tensile steel sheet) having a tensile strength of 590 MPa or greater. “SPC270” means a cold rolled steel sheet (ordinary steel sheet) having a tensile strength of 270 MPa or greater. A steel sheet having a tensile strength of 340 MPa or greater is a high-tensile steel sheet, and steel sheets other than high-tensile steel sheets are ordinary steel sheets.
In a vehicle body side part structure, forming the front section of the side panel outer member using a high-tensile steel sheet eliminates the need for reinforcement (reinforcing material) and allows for a lighter vehicle body side part structure (see Patent Literature 1, for example).
In the vehicle body side part structure disclosed in Patent Literature 1, a high-tensile steel sheet is used in the front section of the side panel outer member. A high-tensile steel sheet becomes harder and more brittle as the tensile strength of the plate is increased. Consequently, in the vehicle body side part structure according to Patent Literature 1, there is a risk of cracks occurring during press forming in round areas with small bending radius, such as at an end line corner of an upper door edge below the side wall of a roof rail, or a corner of a side wall of an upper roof molding groove.
In order to further reduce the weight of the vehicle body side part structure while ensuring vehicle body stiffness during a collision, there is a need to use high-tensile steel sheets having higher tensile strength than SPC590, such as SPC980 or higher-strength steel sheets. Press forming thus becomes more difficult.
In the vehicle body side part structure according to Patent Literature 1, a side sill section protrudes outward so that a fender panel and an outside surface of the side panel outer member are continuous. A level difference therefore occurs on the outside surface of an elbow section between a front pillar lower member and the side sill.
Due to such effects as upward impacts from front wheels during travel, an upper member positioned in front of the front pillar lower member deforms upward, and the front pillar lower member is bent toward the rear. Stress is therefore concentrated at the level difference on the outside surface of the elbow section, and the front pillar lower member folds inward or outward.
Patent Literature 2 discloses a vehicle body side part structure provided with a side sill joined so that a closed section is formed by a side sill outer member and a side sill inner member. A reinforcing member (jack-up stiffener) for preventing deformation of the side sill inner is provided to a jack-up point of the side sill. This vehicle body side part structure makes it possible to prevent the side sill outer member and side sill inner member from deforming.
At the elbow section (connecting section) between the front pillar lower member and the side sill in Patent Literature 1, an intersection occurs between loads directed horizontally in the longitudinal direction of the side sill and loads directed vertically along the front pillar. Consequently, repeated expansion and contraction occur along the joining surface of the side panel outer member and the side panel inner member, thereby adversely affecting the stability and controllability of the vehicle.
In the vehicle body side part structure according to Patent Literature 2, a reinforcing member is provided in the side sill. Although the side sill inner member can therefore be prevented from deforming, the reinforcing member does not contribute to the strength and stiffness of the elbow section connecting the side sill and the front pillar lower member.